1. Field of the Invention
The present invention relates to a method of and apparatus for determining a unknown capacity of a primary or secondary battery by measuring and analyzing a voltage response signal based on a low-frequency pulse current of the battery. More particularly, it relates to a method of and apparatus for measuring battery capacity using a voltage response signal based on a pulse current, which provides a non-destructive measurement of battery capacity, for example, measuring a unknown remaining capacity of a standardized battery product, or in case of a manufacture, grading the rated capacity of the batteries.
2. Discussion of Related Art
A general method used for measuring battery capacity includes a real-time discharge method, which is to measure the usable time t.sub.d for a supply of electrical energy by the battery at a constant current I.sub.d.
If the capacity is expressed in ampere-hour (Ah), the usable time td that a battery supplies electrical energy is given by Equation 1, which is used as the standard for evaluating battery capacity, for example, the Korean Industrial Standards. EQU t.sub.d =Ah/I.sub.d [Equation 1]
A real-time discharge method is one for measuring battery capacity in a direct way and takes a long time throughout the discharge period in real time.
Also, a need of using a plurality of apparatuses arises to discharge multiple batteries independently if the user has to measure plural batteries in a simultaneous manner. This results in an inefficiency in the manufacture of batteries.
A more efficient method used for measuring battery capacity is one that enables a measurement of the battery characteristic for a short time relative to the discharge period of the battery in real-time, as a result of which accurate information can be obtained concerning charge/discharge condition or remaining capacity of the battery.
In regards to battery characteristics involved in the charge/discharge condition, there are used well-known methods for measuring open circuit voltage, voltage and its variations of battery in operation, output signal characteristic responsive to input voltage or current applied to battery, and thereby induced internal resistance or impedance function.
A use of these various methods provides a measurement of battery capacity for a short time relative to a real-time discharge method.
It is however necessary to provide a precise correlation between the measured value and the actual capacity in order to obtain battery capacity in the above-stated methods.
U.S. Pat. No. 3,808,487 discloses a method for sensing the charge condition of a storage battery with a response signal based on a pulse signal which has been periodically applied to the battery during the charge period.
According to the method, information concerning the charge condition and battery capacity is not extracted from the response signal but by a detection of changes in the measured signal, which is expected at the end point of charge.
In another method stated in U.S. Pat. No. 4,952,862, the remaining capacity can be calculated from the measured voltage and the discharge characteristic which is expressed in voltage-hour function including the Peukert parameter.
Especially, EP 119,547 discloses a method used for measuring discharge voltage as a function of time and determining the discharge condition from the averaged change rate of discharge voltage in a predetermined time interval.
When calculating the capacity from a measured voltage and its variants as described above, a precision of the correlation between the measured value and battery capacity is largely dependant upon the discharge characteristic.
For instance, a battery having a great uniformity of voltage provides extremely insignificant variations in the voltage against a change of discharge condition.
Therefore, the method is not considered to be a proper measurement for sensing the discharge condition of a battery.
In order to use a method disclosed in EP 119,547, the precision of a correlation between the measured value and battery capacity must be enhanced by increasing a period of time for measurement or discharge current, which arises an important problem of deterioration in the efficiency of measurement.
For a close correlation with battery capacity, it is very important to measure characteristics related to physical or chemical factors highly affected by the charge/discharge condition of battery, such as internal resistance or impedance.
U.S. Pat. No. 4,678,998 describes a method for examining the charge/discharge condition of battery using a correlation between the remaining capacity and the internal impedance at a specified frequency. This method has been proposed for the users to sense the charge/discharge condition of an automobile battery continuously.
In addition, there are various methods for determining battery capacity with two complex impedances each measured at a high or low frequency (see. U.S. Pat. No. 4,743,855), or with a linear impedance characteristic at a low frequency (see. U.S. Pat. Nos. 5,241,275 and 5,717,336).
An impedance having a close correlation with the charge/discharge condition of a battery is observed at a low frequency of several mHz.
Impedance characteristic in a low frequency band is expressed with a simple equivalent circuit composed of several resistances and condensers. Model parameters of the equivalent circuit are calculated from the measured complex impedance spectrum.
According to the present invention, a voltage signal responsive to pulse current is analyzed to obtain equivalent circuit model parameters of a low-frequency impedance.
A method using pulse signals provides the same model parameters as obtained in an impedance measuring method to determine equivalent circuit impedance model parameters at a low frequency.
The present invention uses a simple apparatus including a current generator for applying a pulse current, a voltameter for measuring the output voltage, a control unit of the voltameter, and an algorithm, as a result of which a similar time is taken in a measurement but more efficiency can be provided than the above-mentioned conventional methods.
Especially, the present invention presents a greatly efficient method and apparatus in a manufacture of battery products, since when measuring a plurality of batteries at the same time, a charging/discharging device used in measuring battery capacity in a real time can be reused or partly modified in use.